The new Third Generation Partnership Project (3GPP) standard known as Long Term Evolution (LTE) (3GPP LTE Rel-10) supports heterogeneous networks. In heterogeneous networks, a mixture of cells of different size and overlapping coverage areas are deployed. For example, a heterogeneous network may deploy pico cells served by relatively low power nodes within the coverage area of a macro cell served by relatively high power base stations. Heterogeneous networks could also deploy relatively low-power home base stations and relays to provide improved service in indoor areas. The aim of deploying low power nodes, such as pico base stations, home base stations, and relays, within a macro cell where coverage is provided by a high power base station is to improve system capacity by means of cell splitting gains, as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployment schemes represent one alternative to deployment of denser networks of macro cells and are particularly effective to cover traffic hotspots, i.e., small geographical areas with high user densities served by lower power nodes.
In heterogeneous networks, there may be a large disparity in output power of the low power nodes compared to the base stations serving macro cells. For example, the output power of the base stations in the macro cells may be in the order of 46 dBm, while the output power of the low power nodes in the pico cells may be less than 30 dBm. In some heterogeneous networks, the macro cells and pico cells operate on the same carrier frequencies and inter-cell interference coordination (ICIC) techniques are used to deal with interference when mobile terminals are operating in areas served by both macro cells and pico cells. For example, scheduling restrictions may be applied where a macro base station is prevented from transmitting in certain subframes, which may be referred to as “blank” subframes or restricted subframes. The pico cells can use these “blank” subframes to transmit data to mobile terminals operating in a link imbalance zone near the cell edge of the pico cells without interference from the macro base stations.
With the introduction of blank subframes in which the macro base station avoids scheduling downlink transmissions, mobile terminals close to the pica base station can be scheduled in all subframes. However, mobile terminals in the link imbalance zone receive signals with large signal strength variations between subframes. For instance in unrestricted subframes (where macro base stations are transmitting on downlink), pilot symbols and data transmitted from the pico cell could be up to 10 dB weaker than the pilots from the macro cell. If the mobile terminal adapts receiver settings based on these weak signals, bad reception performance might be the result.